The present invention relates to apparatus and method for chemical/mechanical polishing for planarizing the surface of a film, such as a conductor film or an insulator film, deposited on a semiconductor substrate in multilevel interconnect and/or element isolation processes of a semiconductor integrated circuit.
Chemical/mechanical polishing (CMP) enables global planarization of a substrate, which cannot be accomplished by any other planarization technique such as resist etch-back. Thus, CMP is one of most noticeable planarization techniques suitably employed for fabricating a semiconductor integrated circuit being miniaturized day by day. In addition, by performing CMP, various problems, such as inaccurate exposure resulting from the variation in depths of focus during a lithography process and inferior reliability of wires formed on a non-planarized surface, can be solved.
A conventional chemical/mechanical polishing apparatus (hereinafter, simply referred to as a xe2x80x9cCMP polisherxe2x80x9d) will be described with reference to FIG. 10. FIG. 10 is a schematic representation illustrating an arrangement for a conventional CMP polisher.
As shown in FIG. 10, a substrate 1 to be polished, made of silicon or the like, is held by a substrate holder 2, which is rotatable and vertically movable. A polishing pad 3 for polishing the surface of the substrate 1 is attached to the planar surface of a polishing platen 4 for moving rotationally. An abrasive (in a slurry state) 5 is supplied through an abrasive supply tube 6 every time by a predetermined amount and dripped onto the polishing pad 3.
In the CMP polisher having such an arrangement, when the polishing platen 4 is rotated with the abrasive 5 dripped through the abrasive supply tube 6 onto the polishing pad 3, the polishing pad 3 is also rotated correspondingly. And when the substrate holder 2 is brought down while rotating, then the substrate 1, held by the substrate holder 2, comes into contact with the polishing pad 3. As a result, the surface of the substrate 1 is polished. The CMP polisher shown in FIG. 10 includes a single substrate holder 2. Accordingly, the polisher is of the type polishing a single substrate 1 during a single polishing process step. Alternatively, if a CMP polisher having a plurality of substrate holders 2 is used, then a plurality of substrates 1 can be polished in parallel with each other during a single polishing process step.
However, if polishing is performed by getting a large number of substrates 1 into contact with the polishing pad 3 one after another, then the polishing surface of the polishing pad 3 gradually loses its capacity to hold the abrasive 5. This is because as polishing is performed for a longer and longer time, the polishing surface of the polishing pad 3 gets more and more clogged owing to the deposition of polishing debris, the mass of abrasive particles and the like. As a result, the amount of the abrasive 5, held in a polishing region where the polishing pad 3 and the substrate 1 are in contact with each other, decreases, and consequently the number of abrasive particles contained in the abrasive 5 also decreases. Accordingly, a rate at which the substrate 1 is polished (hereinafter, simply referred to as a xe2x80x9cpolishing ratexe2x80x9d) adversely decreases.
Thus, it is necessary to re-increase and stabilize the polishing rate by rejuvenating the clogged polishing pad 3 through dressing. xe2x80x9cDressingxe2x80x9d is a process step for recovering the polishing pad""s 3 capacity to hold the abrasive 5 by eliminating clogging from the polishing pad 3. Clogging can be eliminated, for example, by rotating and pressing a dresser 7, to which fine particles of diamond or the like are embedded, against the polishing pad 3. If dressing is performed at regular intervals, then the polishing rate for a substrate can be increased and the variation in polishing rates among substrates can be reduced.
In general, the dressing process step is performed every time a number of substrates have been polished over a predetermined amount of time or every time the number of substrates polished has reached a predetermined number. Also, the dressing process step is performed either in parallel with the polishing process step of a substrate or in an interval between the polishing process steps of substrates.
The amount and number of abrasive particles, which exist on a polishing pad and contributing to polishing, are variable depending upon the roughness of the polishing surface of the polishing pad. Accordingly, the polishing rate of a substrate is also considerably affected by the variation in roughness of the polishing surface of the polishing pad. Thus, in order to keep a polishing rate constant, the roughness of the polishing surface of the polishing pad is desirably kept constant.
However, no method has heretofore been suggested for sensing the roughness of the polishing surface of a polishing pad. Accordingly, as described above, dressing is performed every time a number of substrates have been polished over a predetermined amount of time or every time the number of substrates polished has reached a predetermined number.
Since the roughness of the polishing surface of a polishing pad cannot be kept constant, various inconveniences are very likely to occur in the case of sequentially polishing a large number of substrates. For instance, the polishing rates are gradually decreased or varied among the substrates because the polishing pad gets clogged. Also, since the surface of the polishing pad is glazed, trouble tends to happen in transporting a substrate being held on a substrate holder. Specifically, it becomes less easy to take away the substrate from the polishing pad.
In view of the above-described problems, the present invention was made to reduce the variation in polishing rates among substrates by sensing the roughness of the polishing surface of a polishing pad and by adaptively dressing the polishing pad in accordance with the roughness sensed.
The present inventors supposed that the roughness of the polishing surface of a polishing pad might be sensed based on the rotation torque of a polishing platen on which the polishing pad is fixed. Based on this supposition, we examined the relationship between the polishing rate of a substrate and the rotation torque of a polishing platen from various angles. Herein, the rotation torque of a polishing platen is moment of force about the rotation axis of the polishing platen. Assume the position vector at a point about the rotation axis to be r and the vector of rotational driving force starting from the point to be A. Then, the rotation torque T is given by the vector product of r and A, that is to say, T=rxc3x97A.
In chemical/mechanical polishing, the magnitude of the position vector r is constant, while the magnitude of the rotational driving force vector A is proportional to the frictional force between a polishing pad and a substrate. And the direction of the rotational driving force vector A is aligned with the direction of rotation of the polishing platen, i.e., the rotation direction of the polishing pad. Thus, the rotation torque of the polishing platen is proportional not only to the magnitude of the rotational driving force vector A but also to the frictional force between the polishing pad and the substrate. Accordingly, if the rotation torque of the polishing platen is monitored, then the frictional force between the polishing pad and the substrate and therefore the roughness of the polishing surface of the polishing pad can be sensed nondestructively and instantaneously.
FIG. 11 illustrates the waveform of a signal obtained by quantifying the rotation torque (i.e., a rotation torque signal waveform) of a polishing platen during the polishing process step of a single substrate. In this example, dressing is performed on a polishing pad just before the polishing process step of the substrate is started. As can be understood from FIG. 11, immediately after polishing is started, large rotation torque is obtained thanks to the effect of dressing on the polishing pad. As polishing is performed for a longer and longer time, the effect of dressing attenuates. As a result, the rotation torque decreases to a certain magnitude. However, if an abrasive is continuously supplied, a sufficient amount of abrasive (i.e., a sufficient number of abrasive particles) continuously exists on the polishing surface of the polishing pad. Consequently, rotation torque of constant magnitude is maintained. The rotation torque is a vector and thus has a direction. In FIG. 11, the rotation torque signal waveform is located on the negative domain. This is because the direction of rotation of the polishing platen is clockwise with respect to that of the polishing pad. The direction of rotation torque has nothing to do with the frictional force between the polishing pad and the substrate. It is the absolute value of rotation torque that does have something to with the frictional force. Accordingly, in this specification, the magnitude of rotation torque is represented by the absolute value thereof.
FIG. 12 illustrates the rotation torque signal waveforms of a polishing platen where a plurality of substrates are sequentially polished one after another and where dressing is performed on the polishing pad just before polishing of a substrate is started. As shown in FIG. 12, the rotation torque signal waveforms of the polishing platen for respective substrates have amplitudes gradually decreasing as the continuous polishing process for the substrates proceeds. In other words, as the continuous polishing process is performed for a longer and longer time, the rotation torque gradually decreases. The rotation torque decreases partly because the number of abrasive particles contributing to polishing decreases as the polishing surface of the polishing pad gets more and more clogged.
FIG. 13 illustrates the relationship between the number of substrates processed and a polishing rate. Herein, the polishing rate means a decrease in thickness of a film in a predetermined amount of time. As can be understood from FIG. 13, as the continuous polishing process advances, the polishing rate decreases. The decrease in polishing rates corresponds to the decrease in amplitudes of the rotation torque signal waveforms shown in FIG. 12. In polishing a large number of substrates one after another, the polishing surface of the polishing pad gets more and more clogged as the polishing process advances. As a result, the rotation torque decreases, and the polishing rate also decreases correspondingly.
The present invention was made from these points of view. Specifically, the present invention is embodied in the apparatus and method for chemical/mechanical polishing summarized below.
A first chemical/mechanical polishing apparatus according to the present invention includes: a polishing platen mounted to be rotatable; a polishing pad fixed on the polishing platen; abrasive supply means for supplying an abrasive onto the polishing pad; a substrate holder, mounted to be rotatable above the polishing pad, for holding a substrate to be polished and pressing and polishing the substrate against the polishing pad; a dresser, mounted to be rotatable above the polishing pad, for dressing the polishing pad; torque detection means for detecting at least one of the rotation torque of the polishing platen and the rotation torque of the substrate holder; and dresser control means for making the dresser dress the polishing pad if the rotation torque detected by the torque detection means is equal to or smaller than a predetermined value.
In the first chemical/mechanical polishing apparatus, when clogging is generated in the polishing surface of the polishing pad after a certain number of substrates have been polished one after another, the rotation torque detected by the torque detection means becomes a predetermined value or less because of the decrease in frictional force between the substrate and the polishing pad. Then, the dresser control means drives the dresser to dress the polishing pad. As a result, clogging can be eliminated from the polishing surface of the polishing pad and the amount of the abrasive interposed between the substrate and the polishing pad can be increased. Accordingly, it is possible to prevent the polishing rate from decreasing and to eliminate the variation in polishing rates among the substrates.
A second chemical/mechanical polishing apparatus according to the present invention includes: a polishing platen mounted to be rotatable; a polishing pad fixed on the polishing platen; abrasive supply means for supplying an abrasive onto the polishing pad; a substrate holder, mounted to be rotatable above the polishing pad, for holding a substrate to be polished and pressing and polishing the substrate against the polishing pad; a dresser, mounted to be rotatable above the polishing pad, for dressing the polishing pad; torque detection means for detecting at least one of the rotation torque of the polishing platen, the rotation torque of the substrate holder and the rotation torque of the dresser; and dresser control means for increasing at least one of processing parameters including revolving speed of the dresser, pressure of the dresser against the polishing pad and amount of time during which the dresser dresses the polishing pad if the rotation torque detected by the torque detection means is smaller than a predetermined value.
In the second chemical/mechanical polishing apparatus, when clogging is generated in the polishing surface of the polishing pad after a certain number of substrates have been polished one after another, the rotation torque detected by the torque detection means becomes smaller than a predetermined value because of the decrease in frictional force between the substrate and the polishing pad. Then, the dresser control means increases at least one of the processing parameters including: revolving speed of the dresser; pressure of the dresser against the polishing pad; and amount of time during which the dresser dresses the polishing pad. As a result, clogging can be eliminated from the polishing surface of the polishing pad and the amount of the abrasive interposed between the substrate and the polishing pad can be increased. Accordingly, it is possible to prevent the polishing rate from decreasing and to eliminate the variation in polishing rates among the substrates.
The first or second chemical/mechanical polishing apparatus preferably further includes polishing control means for obtaining a rotation torque integrated value by integrating the rotation torque detected by the torque detection means with respect to time, and for stopping the operation of pressing and polishing the substrate, held by the substrate holder, against the polishing pad when the rotation torque integrated value reaches a prescribed value.
In such an embodiment, the variation in polishing amounts among the substrates can be reduced because a rotation torque integrated value corresponds to a polishing amount of a substrate. As a result, polishing can be performed just as originally designed.
A first chemical/mechanical polishing method according to the present invention includes the steps of: a) rotating a substrate holder, which holds a substrate thereon, with an abrasive supplied onto a polishing pad fixed on a rotating polishing platen, bringing the substrate down to be closer to the polishing pad, and then pressing the substrate against the polishing pad, thereby polishing the substrate; b) detecting at least one of the rotation torque of the polishing platen and the rotation torque of the substrate holder; and c) dressing the polishing pad if the rotation torque detected in the step b) is equal to or smaller than a predetermined value.
In the first chemical/mechanical polishing method, when clogging is generated in the polishing surface of the polishing pad after a certain number of substrates have been polished one after another, the rotation torque detected in the step b) becomes a predetermined value or less because of the decrease in frictional force between the substrate and the polishing pad. Then, the polishing pad is dressed. As a result, clogging can be eliminated from the polishing surface of the polishing pad and the amount of the abrasive interposed between the substrate and the polishing pad can be increased. Accordingly, it is possible to prevent the polishing rate from decreasing and to eliminate the variation in polishing rates among the substrates.
A second chemical/mechanical polishing method according to the present invention includes the steps of: a) rotating a substrate holder, which holds a substrate thereon, with an abrasive supplied onto a polishing pad fixed on a rotating polishing platen, bringing the substrate down to be closer to the polishing pad, and then pressing the substrate against the polishing pad, thereby polishing the substrate; b) dressing the polishing pad by pressing a rotating dresser against the polishing pad; c) detecting at least one of the rotation torque of the polishing platen, the rotation torque of the substrate holder and the rotation torque of the dresser; and d) increasing at least one of processing parameters including revolving speed of the dresser, pressure of the dresser against the polishing pad and amount of time during which the dresser dresses the polishing pad if the rotation torque detected in the step c) is smaller than a predetermined value.
In the second chemical/mechanical polishing method, when clogging is generated in the polishing surface of the polishing pad after a certain number of substrates have been polished one after another, the rotation torque detected in the step c) becomes smaller than a predetermined value because of the decrease in frictional force between the substrate and the polishing pad. Then, at least one of the processing parameters consisting of: revolving speed of the dresser; pressure of the dresser against the polishing pad; and amount of time during which the dresser dresses the polishing pad is increased. As a result, clogging can be eliminated from the polishing surface of the polishing pad and the amount of the abrasive interposed between the substrate and the polishing pad can be increased. Accordingly, it is possible to prevent the polishing rate from decreasing and to eliminate the variation in polishing rates among the substrates.
The first or second chemical/mechanical polishing method preferably further includes the step of obtaining a rotation torque integrated value by integrating the rotation torque detected in the step c) with respect to time, and stopping the operation of polishing the substrate in the step a) when the rotation torque integrated value reaches a prescribed value.
In such an embodiment, the variation in polishing amounts among the substrates can be reduced because a rotation torque integrated value corresponds to a polishing amount of a substrate. As a result, polishing can be performed just as originally designed.
In the second chemical/mechanical polishing method, if the rotation torque detected in the step c) is substantially equal to the predetermined value, the processing parameters are preferably not changed in the step d).
In such an embodiment, the process can swiftly proceed to dressing on the polishing pad.
In the second chemical/mechanical polishing method, the step d) preferably includes the step of further increasing the increased processing parameter if the rotation torque detected in the step c) is still smaller than the predetermined value after the processing parameter has been increased.
In such an embodiment, even if the polishing surface of the polishing pad gets clogged again after polishing on the substrates has further advanced, the polishing surface of the polishing pad can recover its initial state. Accordingly, it is possible to prevent the variation in polishing rates among the substrates.